The RFID (Radio Frequency Identification) system has been known as one of wireless communication systems. Such an REID system generally includes a radio frequency tag (also referred to as an RFID tag) and a reader-writer (RW) apparatus, wherein information is read from or written into the radio frequency tag from the RW apparatus by means of wireless communication.
Known radio frequency tags include one type of tags that can operate by using a power source embedded in the radio frequency tag (such a type is referred to as “active tags”) and another type of tags that operate by using radio waves received from a RW apparatus as driving power (such a type is referred to as “passive tags”).
In an RFID system using a passive tag, the radio frequency tag operates an embedded integrated circuit, such as an IC, an LSI, or the like, using radio signals from an RW apparatus as driving power, and performs various processing in accordance with received radio signals (control signals). The transmission from the radio frequency tag to the RW apparatus is achieved using reflected waves of the received radio signals. That is, various information, such as a tag ID or results of the processing, is carried on the reflected waves, which is sent to the RW apparatus.
Note that a variety of frequency bands have been used for RFID systems, and recently the UHF band (860 MHz-960 MHz) is attracting attentions. The UHF band enables long distance communications, compared to the 13.56 MHz band or the 2.45 GHz band that have been conventionally used. Frequencies around 868 MHz, 915 MHz, and 953 MHz are used in Europe, in the United States, and in Japan, respectively. The communication ranges of radio frequency tags (hereinafter, simply referred to as “tags”) in the UHF band are about between 3 meters and 5 meters, although they depend on an integrated circuit, such as an IC chip, an LSI, or the like, used in the tags. In addition, the output powers of RW apparatuses are about one watt (W).
Note that conventional radio frequency tags include those disclosed in Patent Reference 1 and Patent Reference 2 that are listed below, for example.
Patent Reference 1 discloses a planar antenna that can simplify the structure and reduce the cost, wherein the planar antenna includes a cut-out section that is formed by cutting the base plate into a certain shape from a certain edge. The folded structure is provided only in the cut-out section, which can reduce impedance, enabling matching to a 50Ω feed line without requiring additional circuits, such as an impedance conversion circuit.
Patent Reference 2 discloses a radio frequency tag including a planar antenna formed from a pair of antenna patterns and an IC chip connected to a feeding point of the planar antenna for the purpose of achieving a broader band coverage while reducing the antenna impedance in the radio frequency tag. In the antenna patterns forming the planar antenna, the increased pattern width at the sides distant from the feeding point is formed as a surface pattern with respect to the edge of the feeding point, thereby achieving a broader band coverage of the planar antenna (covering a width of 89 MHz). At the same time, an auxiliary pattern formed adjacent to the planar antenna is formed as a surface pattern having the same area as one antenna pattern of the planar antenna, instead of forming as a linear antenna for reducing the antenna impedance.
Typical sheet-like radio frequency tags that are designed to be attached to a corrugated cardboard or a plastic surface have pass bandwidths of about 200 MHz, and thus can cover the operating frequencies used in all of Europe, the United States, and Japan. However, metal-resistant tags that can be attached on a metal surface have quite narrow pass bands and are designed to be country-specific.
For example, if a planar antenna that has the shape depicted in FIG. 16 and has the communication range characteristic with respect to frequency as depicted in FIG. 15, when the central frequency is adjusted to the operating frequency used in the United States (US), the communication range is extremely dropped at the operating frequencies used in the Europe (EU) and Japan (JP) which are located at two ends. When the central frequency is adjusted to the operating frequency in Europe or Japan, the communication range is similarly extremely dropped at operating frequencies of other regions. In addition, even when used within the same country, the communication range of tags is dropped due to the shift of the frequency characteristic if the tags are attached to a curved surface, or the permittivity (∈r) or the thickness (t) of dielectric substrates (spacer substrates) that are included in the tags.
Accordingly, tags that can be attached on a metal surface and has a broader band frequency characteristic covering all of the operating frequencies used in Europe, the United States, and Japan are desired.
Although patch antennas are typically used for such metal-resistant tags, it is also possible to arrange multiple patch antennas having different sizes in order to achieve a broader band coverage, for example. Non-Patent Reference 1 that is described later discloses an example of such a configuration, although it is not for an RFID tag.
According to Non-Patent Reference 1, when multiple patch antennas are arranged on the same plane, as depicted in FIG. 1 of Non-Patent Reference 1, in order to prevent the patch antennas from interfering with each other, the patch antenna are required to be spaced apart with a spacing of at least half wave (0.5λ).
Patent Document 1: Japanese Laid-Open Patent Publication No. 2006-140735
Patent Document 2: Japanese Laid-Open Patent Publication No. 2006-109396
Non-Patent Reference 1: Desai, B.; Gupta, S., “Dual-band microstrip patch antenna”, Microwave, Antenna, Propagation and EMC Technologies for Wireless Communications, 2005. MAPE 2005. IEEE International Symposium on Volume 1, 8-12 Aug. 2005 Page (s): 180-184 Vol. 1